<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="n1">
<input type="text" id="n2">
<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">
<div></div>
<script>
  function f(x) {
    let num1 = document.querySelector("#n1");
    let num2 = document.querySelector("#n2");
    let d = document.querySelector("div")
    if (isNaN(num2.value)||isNaN(num1.value)){
      d.innerText="";
      return;
    }
    switch (x){
      case 1:
        d.innerText = num1.value*1+num2.value*1;
        break;
      case 2:
        d.innerText = num1.value*1-num2.value*1;
        break;
      case 3:
        d.innerText = num1.value*num2.value;
        break;
      case 4:
        d.innerText = num1.value/num2.value;
        break;
    }
  }
</script>
</body>
</html>